Second-order Raman scattering from linear carbon chains inside multiwalled carbon nanotubes

The present investigation concerns multiwalled carbon nanotubes synthesized on graphite cathodes by arc discharge in a He atmosphere, either with the insertion of a catalytic Ni-Cr mixture or without catalysts. The morphology of the deposited cathodes was investigated by SEM, while the amount of carbon chains inside multiwalled carbon nanotubes (C@MWCNT) in various regions of the deposited cathodes was revealed by a parallel micro-Raman study, through the analysis of the signal from the Raman bands generated by C@MWCNT in the range 1780-1870 cm(-1) (L bands). In samples obtained by using the catalyst, a very high concentration of linear carbon chains is found in some zones, as indicated by the intensity of the L band, which turns out remarkably stronger than the G band of the host nanotubes. In these zones, the second-order Raman scattering is clearly observed, too. The experimental wavenumber values of the 2 L overtone are slightly lower than the exact doubling of the one-phonon peak wavenumber, and this fact is discussed in terms of the existing theoretical predictions for the chain-mode dispersion curve.